Photocurable resins are of continuing interest because they are regarded as ‘green’ solutions, neither requiring solvents nor, if containing water, energy intensive water drying resources. Within this area, it is of growing interest to provide stable photo curable resin compositions which after cure at high speed, result in cured materials with high toughness and increased thermal properties. Such desired properties are particularly sought in three dimensional printing applications.
Liquid-based solid imaging, for example, stereolithography, is a process whereby a photoformable liquid is applied as a thin layer to a surface and exposed to actinic radiation such that the liquid solidifies. Subsequently, new thin layers of photoformable liquids are coated onto previous layers of liquid or previously solidified sections. The new layers are then exposed imagewise in order to solidify portions imagewise and in order to induce adhesion between portions of the new hardened region and portions of the previously hardened region. Each imagewise exposure is of a shape that relates to a pertinent cross-section of a photohardened object such that when all the layers have been coated and all the exposures have been completed, an integral photohardened object can be removed from the surrounding liquid composition.
Initiation of polymerisation in a monomer, oligomer or prepolymer may be effected in a number of ways. One such way is by irradiation, for example with ultraviolet radiation, in which case it is normally necessary that the polymerisable composition should contain an initiator, commonly referred to as a “photoinitiator”. There are two main types of curing chemistry which can be used in this process: free radical and cationic. Although cationic curing has many advantages, its disadvantages, particularly with regard to the photoinitiators used, leads it to be used only in a minority of applications. Most frequently used cationic initiators are either organic iodonium or sulfonium salts.
Photocurable compositions used in the art tend to be thermally instable. The viscosity of photocurable compositions used in the art rises over time, even in absence of UV light, due to thermal decomposition of the photoinitiator. Great efforts are undertaken to stabilise the resins.
Reactive cationic photoinitiators are particularly responsible for viscosity instability in the bath of resin even at 25 to 30° C. or, more especially, if used at elevated temperatures. From the very process of stereolithography, the resin is regularly subjected to low levels of UV-irradiations that triggers photodecomposition of the photoinitiator and produces small amounts of active species. Cationic photoinitiator containing hexafluoroantimony salt or iodonium salts are especially known to be prone to instability, due to their high reactivity. Several attempts have been made to stabilise the formulations by adding various types of low basicity compounds.
U.S. Pat. No. 6,099,787 discloses a process for the production of three-dimensional articles by stereolithography comprising polymerizing of a radiation curable composition comprising a mixture of a cationically curable compound and a free radically curable compound and at least one photoinitiator for polymerization; here, benzyl-N,N-dimethylamine is brought into contact with the composition at a concentration of 5 to 5000 ppm to delay or prevent a significant increase in viscosity of the overall composition.
WO 03/104296 A1 describes an actinic radiation curable composition comprising at least one actinic radiation curable cationically polymerizable compound, at least one cationic photoinitiator and at least one stabilizer which is a complex of a Lewis acid and a Lewis base. The stabilizer is added to the photocurable composition in order to improve storage stability.
U.S. Pat. No. 5,665,792 discloses stabilizers for photo hardenable epoxy composition which have limited solubility in the composition and a density which is different from that of the composition, and which are salts of a group IA or group IIA metal ion and a weak acid, the weak acid having a pKa in water of greater than 3.0.
There is thus a need to further increase the viscosity stability of photocurable liquid compositions. The problems associated with the stabilizer presently known to improve viscosity and thermal stability are that they are consumed to avoid the resin ageing. When the stabilizer is totally consumed, the resin viscosity drastically increases and becomes too high for convenient use. Furthermore, amine stabilizers such as benzyl N,N-dimethylamine decrease the speed of the photocuring process since the amine is basic and consumes the photo acid generated by the cationic photoinitiator during irradiation. By consuming the active species the speed of the photocuring process of the resin is decreased. For that reason only small amounts of amine stabilizer can be added with the consequence that the efficiency is limited.
Cationic photoinitiators containing sulfonium hexafluorophosphate salts are considered to be more stable over time. However, the sulfonium hexafluorophosphate salts are less reactive which reduces the speed of the photocuring process.
WO 00/63272 A1, WO 03/093901 A1, US 2006/0231982 A1 and EP 0 848 294 A1 disclose photocurable resin compositions used for photo fabrication of three dimensional objects which comprise at least one cationic photoinitiator.